Dynamic fail-safe biasing circuitry for fault-tolerant applications

ABSTRACT

A biasing circuit includes a differential communication line comprising a first signal line and a second signal line. The biasing circuit further includes a first current source coupled between a power supply and the first signal line. The biasing circuit further includes a first high-precision voltage reference coupled to the first current source, wherein the first high-precision voltage reference outputs a reference voltage that drives a current produced by the first current source. The biasing circuit further includes a second current source coupled to the second signal line and a system ground, wherein the second current source is driven by a voltage supplied by the power supply.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support. The Government hascertain rights in the invention.

BACKGROUND

Signaling interfaces that transmit across long cables between electronichardware (e.g., low-voltage differential signaling (LVDS)) aresusceptible to noise when not actively driven. In some situations, whena signaling interface is inactive (i.e., undriven), noise or cross-talkcouples onto the cable of the inactive interface and can make it appearas though the inactive interface is active. When this occurs, thereceiver can switch autonomously, causing undesirable results. Thisproblem is particularly present in high-reliability, cross-strappedsystems where the primary and redundant driver interfaces are notpowered at the same time, but the primary and redundant receivers arepowered.

Fail-safe bias resistors have been added to the signal lines in order tokeep the input of the receiver at one level and prevent floating due tonoise. Typically, these resistors are included in the input of thereceiver and are connected to a power supply and ground. Fordifferential signals, the high side is pulled up to the level of thepower supply and the low side is pulled down. In some applications,particularly cross-strapped systems, the bias resistors are too weak toovercome the significant noise that may couple onto a long cable. If theresistors are made stronger to compensate for this, often the driverscannot overcome the biasing current, which can affect the normaloperation of the interface when biasing is not needed.

For the reasons stated above and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the specification, there is a need in the art for improvedsystems and methods for dynamic fail-safe biasing circuitry forfault-tolerant applications.

SUMMARY

The Embodiments of the present invention provide methods and systems fora dynamic fail-safe biasing circuit and will be understood by readingand studying the following specification.

In one embodiment, a biasing circuit comprises: a differentialcommunication line comprising a first signal line and a second signalline; a first current source coupled between a power supply and thefirst signal line; a first high-precision voltage reference coupled tothe first current source, wherein the first high-precision voltagereference outputs a reference voltage that drives a current produced bythe first current source; and a second current source coupled to thesecond signal line and a system ground, wherein the second currentsource is driven by a voltage supplied by the power supply.

DRAWINGS

Understanding that the drawings depict only exemplary embodiments andare not therefore to be considered limiting in scope, the exemplaryembodiments will be described with additional specificity and detailthrough the use of the accompanying drawings, in which:

FIG. 1 is a block diagram illustrating an example differentialcommunication system according to one embodiment of the presentdisclosure.

FIG. 2 is block diagram illustrating an example differentialcommunication system according to one embodiment of the presentdisclosure.

FIG. 3 is a block diagram illustrating an example differentialcommunication system according to one embodiment of the presentdisclosure.

FIG. 4 is a block diagram illustrating an example differentialcommunication system according to one embodiment of the presentdisclosure.

FIG. 5 is a flow chart illustrating a method of increasing noise marginsof a differential communication system according to one embodiment ofthe present disclosure.

In accordance with common practice, the various described features arenot drawn to scale but are drawn to emphasize specific features relevantto the exemplary embodiments.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings that form a part hereof, and in which is shown byway of illustration specific illustrative embodiments. However, it is tobe understood that other embodiments may be utilized and that logical,mechanical, and electrical changes may be made. Furthermore, the methodpresented in the drawing figures and the specification is not to beconstrued as limiting the order in which the individual steps may beperformed. The following detailed description is, therefore, not to betaken in a limiting sense.

Optimally, a fail-safe biasing circuit for a differential communicationsystem should be enabled when the driver is inactive, and should bedisabled when the driver is active. These characteristics prevent thefail-safe biasing circuit from interfering with the performance of thedriver, which is a problem with using bias resistors that are toopowerful. Further, the fail-safe biasing circuit should provide enoughnoise margin so any noise or cross-talk that couples onto the cablesdoes not result in autonomous switching at the receiver or otherundesirable results. Previous fail-safe bias circuits did not accomplishthese optimal qualities.

The embodiments described below provide systems and methods for adynamic fail-safe biasing circuit for fault-tolerant applications. Theembodiments described below include current sources connected to a powersupply and the signal lines to provide a consistent current and voltageto inactive differential communication lines. Further, the embodimentsdescribed below include mechanisms for ensuring that the biasing circuitdoes not interfere with normal operation of the differentialcommunication system.

FIG. 1 is a block diagram illustrating an example differentialcommunication system 100 according to one embodiment of the presentdisclosure. System 100 includes a driver 102 (for example, adifferential signal driver) coupled to an optional connector 104, abiasing circuit 105, a first power supply (PS1) 126, and a second powersupply (PS2) 128. In the configuration of FIG. 1, first power supply 126and second power supply 128 are implemented as redundant power supplies.For example, first power supply 126 may be implemented as the primarypower supply while second power supply 128 acts as a backup/redundantpower supply that becomes active upon loss of first power supply 126.Accordingly, the power supplies 126, 128 would be controlled to preventboth from being active at the same time. In exemplary embodiments, thepower supplies 126, 128 would be controlled by a control signal from aninterface, a field programmable gate array, a processor, a power onreset circuit, or some other external controller known to those havingskill in the art. Thus, when the first power supply 126 is disabled, thecontrol signal would turn on the second power supply 128.

The driver 102 comprises an electronics device powered by the secondpower supply 128 and is coupled to the optional connector 104 through ahigh-side signal line 122 and a low-side signal line 124. In theembodiment shown in FIG. 1, driver 102 comprises communicationelectronics that communicate to another device 130 over communicationcable 132 using a differential signal transmitted across signal lines122 and 124. Connector 104 comprises an interface (such as a plug orother termination devices) that physically and electrically couplessignal lines 122 and 124 to signal lines within communication cable 132.In one embodiment, driver 102 comprises a transmitter that sends adifferential signal across communication cable 132. In otherembodiments, driver 102 comprises a receiver that receives adifferential signal from communications cable 132. In alternateembodiments, the system 100 can be implemented using a differentialsignal standard such as LVDS, RS-422, RS-485, or other differentialsignal standards known to those in the art.

As shown in FIG. 1, biasing circuit 105 comprises a first current source106 coupled to high-side signal line 122 and a second current source 108coupled to low-side signal line 124. The second current source 108 isalso coupled to ground. Both the first current source 106 and the secondcurrent source 108 are powered by the first power supply 126. Thisensures that current sources 106, 108 are active at the same time, whichforms a complete circuit to provide a biasing voltage and current to thesignal lines 122, 124. In exemplary embodiments, the first currentsource 106 and the second current source 108 operate as switches. Thecurrent sources 106 and 108 may be implemented using various means. Forexample, in some embodiments, one or both of the current sources 106 and108 comprise bipolar junction transistors. In other embodiments, thecurrent sources are implemented using field-effect transistors,mechanical switches, mechanical relays, or a resistor network. In otherembodiments, the current sources are implemented using another type ofcurrent source known to one having skill in the art.

The first current source 106 is also coupled to a high-precision voltagereference 110, which drives first current source 106 by providing aprecise voltage to control the current supplied by the current source106. In some exemplary embodiments, the high-precision voltage reference110 is a Zener diode. In other embodiments, the high-precision voltagereference 110 is a resistor divider circuit. In other embodiments, thehigh-precision voltage reference 110 can be another type of precisionvoltage reference known to one having skill in the art. As indicated inFIG. 1, in some implementations, the high-precision voltage reference110 may also be coupled to ground.

Biasing circuit 105 may also comprise one or more biasing and/or currentlimiting resistors such as shown by resistors 112, 114, 116, 118 and120. For example, high-precision voltage reference 110 and currentsource 106 are each coupled to first power supply 126 via resistors 112and 114 respectively. Current source 106 is coupled to high side signalline 122 by resistors 116. Current source 108 is coupled to first powersupply 126 via resistor 120 and to low-side signal line 124 viaresistors 118. While a pair of resistors is shown for resistors 116,118, 120, it should be understood that each pair could be replaced witha single resistor.

In operation, communication system 100 operates with either the firstpower supply 126 powered or the second power supply 128 powered, but notboth. When the first power supply 126 is energized, the biasing circuit105 is powered on by the first power supply 126, and the voltagesupplied to the first current source 106 is controlled by thehigh-precision voltage reference 110. Thus, when the first power supply126 is powered on, the high-precision voltage reference 110 turns on thefirst current source 106. Since the driver 102 is powered by the secondpower supply 128, the driver 102 is not powered or active when thebiasing circuit 105 is active. The high-side signal line 122 extendingfrom the driver 102 to the optional connector 104 is coupled to thecurrent source 106. Current source 106 provides a constant voltage andcurrent to high-side signal line 122 through the optional connector 104,which sends the voltage and current through cable 132 to device 130(which may be a receiver, for example). In exemplary embodiments, theranges for the voltage and current vary depending on the systemrequirements. The voltage and current pass through a termination element(which may be a resistor, for example) in device 130, back to theoptional connector 104 via cable 132, and to low-side signal line 124.The voltage and current pass through current source 108 to complete thecircuit. It should be noted that current source 108 does not need to beoperated to provide a voltage or a current to the low-side signal line124. Instead, current source 108 acts as a switch to couple the low-sidesignal line 124 (and therefore the current pushed onto signal line 122by current source 106) to a current sink (such as a system ground, forexample).

In exemplary embodiments, system 100 shown in FIG. 1 is incorporatedinto a cross-strapped differential communication system that includesboth a primary and a redundant side. In such embodiments, the primaryside could be powered by the first power supply 126 and the redundantside could be powered by the second power supply 128. Since the driver102 in FIG. 1 is powered by the second power supply 128, system 100, asshown in FIG. 1, would represent the transmitter of the redundant sideof the cross-strapped differential communication system in suchembodiments. The transmitter of the primary side would include anessentially identical schematic as that of system 100 shown in FIG. 1,with one difference. The first power supply 126 would power the driver102 in the primary side, and the second power supply 128 would power thebiasing circuit 105 in the primary side. As discussed above, the powersupplies 126, 128 are not active at the same time. Since biasing circuit105, in the redundant side, is coupled to the first power supply 126,the biasing circuit 105 would only be active when the primary driver waspowered on. Similarly, the biasing circuit in the primary side wouldonly be active when the driver 102 of the redundant side was powered on.This design ensures that the biasing circuits in the respective sides ofthe cross-strapped differential communication system do not interferewith the normal operation of the drivers.

FIG. 2 is a block diagram illustrating an example differentialcommunication system 200 according to one embodiment of the presentdisclosure. System 200 includes a driver 102, an optional connector 104,a biasing circuit 205, a first power supply (PS1), and a second powersupply (PS2). In exemplary embodiments, biasing circuit 205 includesfour current sources 106, 108, 206, 208 and two high-precision voltagereferences 110, 210. Biasing circuit 205 may also comprise one or morebiasing and/or current limiting resistors such as shown resistors 112,114, 116, 118, 120, 212, 214, 216, 218, 220. The device 130 andconnecting cable 132, shown in FIG. 1, are not shown in FIG. 2 forclarity purposes, but it should be understood that those componentscould be part of system 200. Similar to system 100, in some embodiments,system 200 can be implemented in a cross-strapped differentialcommunication system.

Since system 200 is similar to system 100, only the differences will bediscussed. However, it should be appreciated that like named andnumbered elements discuss in FIG. 1 operate in the same manner withrespect to FIG. 2 with the same options and alternatives, and viceversa. System 200 includes a second current source 206 coupled to thehigh-side signal line 122. Like current source 106, current source 206is coupled to the first power supply 126 and the high-side signal line122 through resistors 214, 216. Current source 206 is also coupled to ahigh-precision voltage reference 210. Current source 206 is connected tothe first power supply 126 and the high-side signal line 122 in parallelwith current source 106. Current source 206 and high-precision voltagereference 210 have characteristics similar to those of current source106 and high-precision voltage reference 110, discussed above withrespect to FIG. 1.

System 200 also includes a second current source 208 coupled to thelow-side signal line 124. Like current source 108, current source 208 isalso coupled to the first power supply 126 and the low-side signal line124. Current source 208 is also coupled to a system ground. Currentsource 208 is connected to the first power supply 126 and a systemground in parallel with current source 108. Current source 208 hascharacteristics similar to those of current source 108, discussed abovewith respect to FIG. 1.

Since each current source 106, 108, 206, 208 is powered by the firstpower supply 126, the current sources 106, 108, 206, 208 will be activeat the same time. When the current sources 106, 108, 206, 208 areactive, this forms a complete circuit to provide a biasing voltage andcurrent to the signal lines. When both current sources 106, 206 areoperational, the current sources 106, 206 share the current and voltagethat is provided to the high-side signal line 122. If there is a failurein one of the current sources 106, 206, the system 200 is capable ofoperation using the current source that is still operational. Thus, thespecifications of the current sources 106, 206 and the high-precisionvoltage reference 110, 210 are determined based on the system 200operating with a single current coupled to the high-side signal line122.

Similarly, when both current sources 108, 208 are operational, thecurrent sources 108, 208 share the current and voltage that is receivedfrom the low-side signal line 124. If there is a failure in one of thecurrent sources 108, 208, the system 200 is capable of operation usingthe current source that is still operational. Thus, the specificationsof the current sources 108, 208 are determined based on the systemoperating with a single current source on the high-side signal line 122.

FIG. 3 is a block diagram illustrating an example differentialcommunication system 300 according to one embodiment of the presentdisclosure. System 300 includes a driver 102, an optional connector 104,a biasing circuit 305, a first power supply (PS1), and a second powersupply (PS2). Biasing circuit 305 comprises one example implementationof biasing circuit 205 illustration where the current sources may beimplemented using transistors and the voltage references may beimplemented using Zener diodes. In exemplary embodiments, biasingcircuit 305 includes four transistors 306, 308, 310, 312 and two Zenerdiodes 314, 316. Biasing circuit 305 may also comprise one or morebiasing and/or current limiting resistors such as shown resistors 318,320, 322, 324, 326, 328, 330, 332, 334, 336. The device 130 andconnecting cable 132, shown in FIG. 1, are not shown in FIG. 3 forclarity purposes, but it should be understood that those componentscould be part of system 300. Similar to system 100, in some embodiments,system 300 can be implemented in a cross-strapped differentialcommunication system.

The transistors 306, 308, 310, 312 in biasing circuit 305 are organizedinto pairs. The first pair of transistors includes transistors 306, 308and the second pair of transistors includes transistors 310, 312. Thefirst pair of transistors 306, 308 are connected to the first powersupply 126 and the high-side signal line 122 in parallel. The secondpair of transistors 310, 312 are connected to the low-side signal line124 and a system ground in parallel. In exemplary embodiments, thetransistors 306, 308, 310, 312 are NPN bipolar junction transistors. Inother embodiments, the transistors 306, 308, 310, 312 are PNP bipolarjunction transistors. While the transistors 306, 308, 310, 312 are shownas bipolar junction transistors in FIG. 3, it should be understood thatdifferent types of transistors could be used in biasing circuit 305. Forexample, the transistors 306, 308, 310, 312 could bemetal-oxide-semiconductor field-effect transistors.

In exemplary embodiments, the minimum threshold values for thetransistors 306, 308, 310, 312 are determined such that system 300 couldcontinue to maintain operation within the operating range even with asingle active transistor in each pair. Thus, although biasing circuit305 is shown with four transistors 306, 308, 310, 312, in someembodiments, biasing circuit 305 would be capable of performingfail-safe biasing for system 300 with only one transistor from eachpair. By organizing the transistors 306, 308, 310, 312 in parallel pairsand selecting appropriate minimum threshold values, biasing circuit 305provides fault-tolerance in the event of a failure in one of thetransistors 306, 308, 310, 312. In exemplary embodiments, biasingcircuit 305 could also be configured to include more than fourtransistors.

The base of each transistor 306, 308 of the first pair is coupled to thecathode of a respective Zener diode 314, 316. The collector of eachtransistor 306, 308 in the first pair is coupled to the first powersupply 126 through a resistor 320, 326. The emitter of each transistor306, 308 in the first pair is coupled to the high-side signal line 122through a pair of resistors 322, 328 connected in series. In exemplaryembodiments, the transistors 306, 308 are biased to operate as switches.

The base of each transistor 310, 312 of the second pair is connected tothe first power supply 126 through a pair of resistors 332, 336 that areconnected in series. The collector of each transistor 310, 312 in thesecond pair is coupled to the low-side signal line 124 through a pair ofresistors 330, 334 that are connected in series. The emitters of eachtransistor 310, 312 in the second pair is coupled to ground. The secondpair of transistors 310, 312 are coupled to the low-side signal line 124in order to complete the circuit through the signal lines 122, 124. Inexemplary embodiments, the transistors 310, 312 are biased to operate asswitches.

Some of the resistors 322, 328, 330, 332, 334, 336 are grouped in pairsin order to increase the fault-tolerance of the system. In exemplaryembodiments, fault-tolerance is not required and the pairs of resistorsare replaced by a single resistor. By organizing the resistors 322, 328,330, 332, 334, 336 in pairs, a single shorted resistor will not hindercontinued operation of the biasing circuit 305. The values of theresistors 318, 320, 322, 324, 326, 328, 330, 332, 334, 336 can beadjusted in order to change the voltage and current provided to thesignal lines 122, 124. In exemplary embodiments, the value of theresistors 322, 328, 330, 334 is 324 ohms. In exemplary embodiments, thevalue of the resistors 320, 326 is 1 kilo-ohm. In exemplary embodiments,the value of the resistors 318, 324 is 4.99 kilo-ohms. In exemplaryembodiments, the value of the resistors 332, 336 is 1.5 kilo-ohms.

The Zener diodes 314, 316 operate as a high-precision voltagereference/regulator for the transistors 306, 308. The anode of eachZener diodes 314, 316 is connected to a system ground. The cathode ofeach Zener diode 314, 316 is connected to the first power supply 126through a resistor 318, 324 connected in series with the Zener diode314, 316. The single resistors 318, 324 limit the current flow throughthe Zener diodes 314, 316.

The biasing circuit 305 is powered on by the first power supply 126. Thevoltage supplied to the base of transistors 306, 308 in the first pairis controlled by the Zener diodes 314, 316. Thus, when the first powersupply 126 is powered on, it biases the Zener diodes 314, 316, whichturn on the transistors 306, 308. Since Zener diodes provide astabilized voltage output over time and varying load, Zener diodesprovide a manageable way of ensuring that the voltage to thedifferential lines will be accurately known. In exemplary embodiments,the Zener diodes 314, 316 are set to provide 2.4 volts to the base ofthe transistors 306, 308, and the base-emitter voltage drop for thetransistors 306, 308 is approximately 0.7 volts. When the first powersupply 126 is powered on, this also turns on the transistors 310, 312.

When all transistors are operational, transistors 306, 308 current shareand transistors 310, 312 current share. When the biasing circuit 305 isactive, transistors 306, 308 provide a constant voltage and current tothe high-side signal line 122, which extends from the driver 102 to theoptional connector 104. The constant voltage and current is providedthrough the optional connector 104, which sends the voltage and currentto the device 130 through cable 132. The voltage and current passthrough the termination resistor in the device 130 and back to theoptional connector 104 through cable 134. The voltage and current passthrough the second pair of transistors 310, 312 to a system groundcomplete the circuit.

FIG. 4 is a block diagram illustrating an example differentialcommunication system 400 according to one embodiment of the presentdisclosure. System 400 includes a driver 102, an optional connector 104,a biasing circuit 405, and a first power supply (PS1) 126. In exemplaryembodiments, biasing circuit 405 includes four transistors 406, 408,410, 412; two Zener diodes 414, 416; and a plurality of resistors 418,420, 422, 424, 426, 428, 430, 432, 434, 436. The device 130 andconnecting cable 132 are not shown in FIG. 4 for clarity purposes, butit should be understood that those components could be part of system400. In some embodiments, system 400 is implemented in ahigh-reliability, single-stream differential communication system.

System 400 functions similarly to system 300 with one major difference.System 300 is implemented using two different (and redundant) powersupplies, whereas system 400 only includes a single power supply. Sincethe driver 102 and the biasing circuit 405 are both powered by the firstpower supply 126, system 400 includes a different mechanism for engagingthe biasing circuit 405 to ensure that the driver 102 and the biasingcircuit 405 are not active at the same time. As discussed above, it isundesirable to have the biasing circuit active at the same time as thedriver because the biasing circuit will interfere with the normaloperation of the driver.

The operation of the components of system 400 is similar to that of thecomponents in system 300, so only the differences between the systemswill be discussed. It should therefore be understood that the variationsdiscussed above with reference to system 300 would also apply to system400.

The driver 102 and the transistors 406, 408 are connected to the firstpower supply 126. In order to prevent the biasing circuit 405 frominterfering with the normal operation of the driver 102, system 400includes a bias circuit controller 450 to enable or disable the biasingcircuit 405 and the driver 102. In exemplary embodiments, the biascircuit controller 450 may comprise, for example, a field programmablegate array, a processor, a power on reset circuit, or some otherexternal controller known to those having skill in the art.

Bias circuit controller 450 provides logic level signals 438 toenable/disable the transistors 406, 408, 410, 412. When the transistors406, 408, 410, 412 are enabled by bias circuit controller 450, thebiasing circuit 405 is activated. When the biasing circuit 405 isactive, the Zener diodes 416, 418 are biased in a manner similar to thatdiscussed above with reference to FIG. 3. Since biasing circuit 405should only be active when the driver 102 is not operating, the logiclevel signal 440 provided to the driver 102 will be opposite the logiclevel signals 438 provided to the transistors 406, 408, 410, 412. Thus,proper operation of system 400 requires that the bias circuit controller450 ensure that the transistors 406, 408, 410, 412, are in an oppositestate compared to the driver 102.

In exemplary embodiments, the bias circuit controller 450, discussedabove with reference to system 400 could be utilized in anyimplementation of the systems 100, 200 and 300 as well. Utilization ofsuch a controller may be beneficial in situations where a higher levelof control over the biasing circuit 104, 205, 305 is desired.

The biasing circuits 105, 205, 305, 405 discussed above with referenceto FIGS. 1-4 can be used to replace the fail-safe bias resistors thatare not always effective for providing adequate noise margins forsingle-stream differential communication systems that experience noise.The biasing circuits 105, 205, 305, 405 prevent the receiver fromautonomously switching due to noise coupling on the cables because thebiasing circuits 105, 205, 305, 405 provide a voltage and current tomaintain the signal to the receiver when the cables are not beingactively driven.

Further, the biasing circuits 105, 205, 305, 405 do not inhibitperformance of the driver that is active because the biasing circuit isonly engaged when the driver is inactive. For biasing circuits 105, 205,305, this is ensured because the biasing circuit and the drivers areactivated by different power supplies. For biasing circuit 405, acontroller can be used to control the biasing circuit 405 and the driver102 to ensure that the biasing circuit 405 is not powered on when thedriver is operational. Thus, biasing circuits 105, 205, 305, 400 providea significant advantage over prior systems that employed fail-safe biasresistors because they will not interfere with the normal operation ofthe drivers.

FIG. 5 is a flow chart illustrating a method 500 for providing fail-safebiasing to a differential communication system, such as differentialcommunication systems 100, 200, 300, 400.

The method begins at 502 with applying a biasing current and biasingvoltage to a first signal line using a first current source powered by afirst power supply. In one embodiment, applying the biasing current andbiasing voltage to the first a signal line may further include turningon the first current source with a reference voltage from a firsthigh-precision voltage reference that is biased when the first powersupply is turned on. In that case, the method at 502 may further includeregulating the biasing current by driving the first current source witha high-precision voltage reference, wherein the high-precision voltagereference is also powered by the first power supply.

The first current source may be coupled between the first signal lineand the first power supply. In other exemplary embodiments, applying thebiasing current and biasing voltage further includes providing a logiclevel signal to the first current source. In exemplary embodiments, thelogic level signal is provided by a controller, such as the bias circuitcontroller 450 discussed above with reference to FIG. 4.

The method proceeds to 504 with coupling a second signal line to acurrent sink using a second current source powered by the first powersupply. In one embodiment, coupling the second signal line may furthercomprise turning on the second current source with a voltage from thefirst power supply. The second current source may be coupled between thesecond signal line and a current sink (such as, for example, a systemground). In exemplary embodiments, coupling the second signal line tothe current sink further includes providing a logic level signal to thesecond current source. In exemplary embodiments, the logic level signalis provided by a controller, such as the bias circuit controller 450discussed above with reference to FIG. 4.

The method then proceeds to 506 wherein an electrical path for thebiasing current between the first signal line and the second signal lineis completed by an electrical device coupled to the first signal lineand the second signal line by at least one cable. As discussed above, adifferential signal driver is coupled to the first signal line and thesecond signal line and configured to transmit a differential signalacross the first signal line and the second signal line. In someembodiments, the differential signal driver is coupled to a second powersupply, wherein the second power supply is de-energized whenever thefirst power supply is energized and vice versa. In that case, thebiasing current through the first signal line and the second signal lineis applied only when the first power supply is energized. This providesa biasing signal to the external electrical device such that noise onthe cable (which may occur when the differential signal driver is not inoperation) becomes negligible as compared to the biasing signal.

As discussed above, when the first power supply is energized, the firstcurrent source is powered on by the first power supply, and the voltagesupplied to the first current source may be controlled by ahigh-precision voltage reference. In that case, when the first powersupply is powered on, the high-precision voltage reference turns on thefirst current source. The voltage and current pass through a terminationelement (which may be a resistor, for example) in the electrical deviceto second signal line. The voltage and current pass through secondcurrent source to the current sink to complete the circuit.

In other embodiments, additional current sources may be included toprovide fault tolerance. In such embodiments, a third current source maybe coupled between the first signal line and the first power supply inparallel with the first current source. Also, the second signal line maybe coupled to the current sink using a fourth current source powered bythe first power supply and connected in parallel with the second currentsource. The first current source and the third current source areconfigured to provide a portion of the biasing current and biasingvoltage applied to the first signal line. The second current source andthe fourth current source are configured to receive a portion of thevoltage and current from the second signal line. In exemplaryembodiments, the third current source and the fourth current source havecharacteristics similar to the first and second current sourcesrespectively.

In some embodiments, the fail-safe biasing is performed in thetransmitter of the differential communication system. In suchembodiments, the biasing current and the biasing voltage are transmittedto the receiver of the differential communication system. In otherembodiments, the fail-safe biasing is performed in the receiver of thedifferential communication system.

Example Embodiments

Example 1 includes a transmitter of a differential communication system,comprising: a differential signal driver coupled to a first powersupply; a differential line comprising a first signal line and a secondsignal line; a fail-safe biasing circuit coupled to a second powersupply, including: a first current source coupled between the secondpower supply and the first signal line; a first high-precision voltagereference directly coupled to the first current source, wherein thefirst high-precision voltage reference outputs a reference voltage thatdrives a current produced by the first current source; and a secondcurrent source coupled to the second signal line and a current sink,wherein the second current source is driven by a voltage supplied by thesecond power supply.

Example 2 includes the differential communication system of Example 1,wherein the fail-safe biasing circuit further includes: a third currentsource coupled between the second power supply and the first signalline, wherein the third current source is connected to the second powersupply and the first signal line in parallel with the first currentsource; a second high-precision voltage reference directly coupled tothe third current source, wherein the second high-precision voltagereference outputs a reference voltage that drives a current produced bythe third current source; and a fourth current source coupled to thesecond signal line and a system ground, wherein the fourth currentsource, wherein the fourth current source is driven by a voltagesupplied by the second power supply, wherein the fourth current sourceis connected to the second power supply and the second signal line inparallel with the second current source.

Example 3 includes the differential communication system of any ofExamples 1-2, wherein the first current source and the second currentsource are each selected from a group consisting of: a field-effecttransistor; a mechanical switch; a mechanical relay; and a resistornetwork.

Example 4 includes the differential communication system of Example 1-3,wherein the first high-precision voltage reference is selected from agroup consisting of: a Zener diode; and a resistor divider circuit.

Example 5 includes the differential communication system of any ofExamples 1-4, wherein the system is configured so only one of the firstpower supply and the second power supply are powered on at a time.

Example 6 includes a biasing circuit, the circuit comprising: adifferential communication line comprising a first signal line and asecond signal line; a first current source coupled between a powersupply and the first signal line; a first high-precision voltagereference coupled to the first current source, wherein the firsthigh-precision voltage reference outputs a reference voltage that drivesa current produced by the first current source; and a second currentsource coupled to the second signal line and a system ground.

Example 7 includes the biasing circuit of Example 6, further comprising:a third current source coupled between the power supply and the firstsignal line, wherein the third current source is connected to the powersupply and the first signal line in parallel with the first currentsource; a second high-precision voltage reference coupled to the thirdcurrent source, wherein the second high-precision voltage referenceoutputs a reference voltage that drives a current produced by the thirdcurrent source; and a fourth current source coupled to the second signalline and a system ground, wherein the second current source and thefourth current source are driven by a voltage supplied by the powersupply.

Example 8 includes the biasing circuit of Example 7, wherein the firstcurrent source, the second current source, the third current source, andthe fourth current source are each selected from a group consisting of:a field-effect transistor; a mechanical switch; a mechanical relay; anda resistor network.

Example 9 includes the biasing circuit of Example 7, wherein the firsthigh-precision voltage reference and the second high-precision voltagereference each comprise a Zener diode.

Example 10 includes the biasing circuit of any of Examples 6-9, whereinthe biasing circuit is implemented in a transmitter of a differentialcommunication system.

Example 11 includes the biasing circuit of any of Examples 6-10, whereinthe biasing circuit is implemented in a receiver of a differentialcommunication system.

Example 12 includes the biasing circuit of any of Examples 6-11, whereinthe biasing circuit is implemented in both a primary side and aredundant side of a cross-strapped, differential communication system.

Example 13 includes the biasing circuit of any of Examples 6-12, whereinthe biasing circuit is implemented in a single-stream differentialcommunication system.

Example 14 includes the biasing circuit of Example 6-13, furthercomprising a controller configured to provide logic level signals toenable or disable the first current source and the second currentsource, wherein the second current source is driven by a voltagesupplied by the controller.

Example 15 includes a method of providing fail-safe biasing for adifferential communication system, comprising: applying a biasingcurrent and biasing voltage to a first signal line using a first currentsource powered by a first power supply; coupling a second signal line toa current sink using a second current source; wherein an electrical pathfor the biasing current between the first signal line and the secondsignal line is completed by an electrical device coupled to the firstsignal line and the second signal line by at least one cable.

Example 16 includes the method of Example 15, wherein applying thebiasing current and biasing voltage further includes providing a firstlogic level signal, with a controller, to enable the first currentsource; and wherein coupling the second signal line to the current sinkfurther includes providing a second logic level signal, with thecontroller, to enable the second current source.

Example 17 includes the method of any of Examples 15-16, whereinapplying the biasing current and biasing voltage to a first signal linefurther includes using a third current source powered by the first powersupply; and wherein coupling the second signal line to the current sinkfurther includes using a fourth current source, wherein the secondcurrent source and the fourth current source are powered by the firstpower supply.

Example 18 includes the method of Example 17, further comprising:coupling the first current source and the third current source inparallel so both the first current source and the third current sourceare configured to provide a portion of the biasing current and biasingvoltage to the first signal line; and coupling the second current sourceand the fourth current source in parallel so both the second currentsource and the fourth current source are configured to receive a portionof a current and a voltage from the second signal line.

Example 19 includes the method of any of Examples 15-18, furthercomprising regulating the biasing current by driving the first currentsource with a high-precision voltage reference, wherein thehigh-precision voltage reference is also powered by the first powersupply.

Example 20 includes the method of any of Examples 15-19, wherein adifferential signal driver is coupled to the first signal line and thesecond signal line and configured to transmit a differential signalacross the first signal line and the second signal line, wherein thedifferential signal driver is coupled to a second power supply, whereinthe second power supply is de-energized whenever the first power supplyis energized.

In various alternative embodiments, system elements, method steps, orexamples described throughout this disclosure (such as the bias circuitcontroller, for example) may be implemented on one or more computersystems, field programmable gate array (FPGA), or similar devicescomprising a processor executing code to realize those elements,processes, or examples, said code stored on a non-transient data storagehardware device. Therefore other embodiments of the present disclosuremay include elements comprising program instructions resident oncomputer readable media which when implemented by such computer systems,enable them to implement the embodiments described herein. As usedherein, the term “computer readable media” refers to tangible memorystorage devices having non-transient physical forms. Such non-transientphysical forms may include computer memory devices, such as but notlimited to punch cards, magnetic disk or tape, any optical data storagesystem, flash read only memory (ROM), non-volatile ROM, programmable ROM(PROM), erasable-programmable ROM (E-PROM), random access memory (RAM),or any other form of permanent, semi-permanent, or temporary memorystorage system or device having a physical, tangible form. Programinstructions include, but are not limited to computer-executableinstructions executed by computer system processors and hardwaredescription languages such as Very High Speed Integrated Circuit (VHSIC)Hardware Description Language (VHDL).

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement, which is calculated to achieve the same purpose,may be substituted for the specific embodiment shown. This applicationis intended to cover any adaptations or variations of the presentinvention. Therefore, it is manifestly intended that this invention belimited only by the claims and the equivalents thereof.

What is claimed is:
 1. A transmitter of a differential communicationsystem, comprising: a differential signal driver coupled to a firstpower supply; a differential line coupled to the differential signaldriver, wherein the differential line comprises a first signal line anda second signal line; a fail-safe biasing circuit coupled to a secondpower supply, including: a first current source coupled between thesecond power supply and the first signal line; a first high-precisionvoltage reference directly coupled to the first current source, whereinthe first high-precision voltage reference outputs a reference voltagethat drives a current produced by the first current source; and a secondcurrent source coupled to the second signal line and a current sink,wherein the second current source is driven by a voltage supplied by thesecond power supply.
 2. The differential communication system of claim1, wherein the fail-safe biasing circuit further includes: a thirdcurrent source coupled between the second power supply and the firstsignal line, wherein the third current source is connected to the secondpower supply and the first signal line in parallel with the firstcurrent source; a second high-precision voltage reference directlycoupled to the third current source, wherein the second high-precisionvoltage reference outputs a reference voltage that drives a currentproduced by the third current source; and a fourth current sourcecoupled to the second signal line and a system ground, wherein thefourth current source is driven by a voltage supplied by the secondpower supply, wherein the fourth current source is connected to thesecond power supply and the second signal line in parallel with thesecond current source.
 3. The differential communication system of claim1, wherein the first current source and the second current source areeach selected from a group consisting of: a field-effect transistor; amechanical switch; a mechanical relay; and a resistor network.
 4. Thedifferential communication system of claim 1, wherein the firsthigh-precision voltage reference is selected from a group consisting of:a Zener diode; and a resistor divider circuit.
 5. The differentialcommunication system of claim 1, wherein the system is configured soonly one of the first power supply and the second power supply arepowered on at a time.
 6. A biasing circuit, the circuit comprising: adifferential communication line comprising a first signal line and asecond signal line; a first current source coupled between a powersupply and the first signal line; a first high-precision voltagereference coupled to the first current source, wherein the firsthigh-precision voltage reference outputs a reference voltage that drivesa current produced by the first current source; and a second currentsource coupled to the second signal line and a system ground.
 7. Thebiasing circuit of claim 6, further comprising: a third current sourcecoupled between the power supply and the first signal line, wherein thethird current source is connected to the power supply and the firstsignal line in parallel with the first current source; a secondhigh-precision voltage reference coupled to the third current source,wherein the second high-precision voltage reference outputs a referencevoltage that drives a current produced by the third current source; anda fourth current source coupled to the second signal line and a systemground, wherein the second current source and the fourth current sourceare driven by a voltage supplied by the power supply.
 8. The biasingcircuit of claim 7, wherein the first current source, the second currentsource, the third current source, and the fourth current source are eachselected from a group consisting of: a field-effect transistor; amechanical switch; a mechanical relay; and a resistor network.
 9. Thebiasing circuit of claim 7, wherein the first high-precision voltagereference and the second high-precision voltage reference each comprisea Zener diode.
 10. The biasing circuit of claim 6, wherein the biasingcircuit is implemented in a transmitter of a differential communicationsystem.
 11. The biasing circuit of claim 6, wherein the biasing circuitis implemented in a receiver of a differential communication system. 12.The biasing circuit of claim 6, wherein the biasing circuit isimplemented in both a primary side and a redundant side of across-strapped, differential communication system.
 13. The biasingcircuit of claim 6, wherein the biasing circuit is implemented in asingle-stream differential communication system.
 14. The biasing circuitof claim 6, further comprising a controller communicatively coupled tothe first current source and the second current source, wherein thecontroller is configured to provide logic level signals to enable ordisable the first current source and the second current source, whereinthe second current source is driven by a voltage supplied by thecontroller.
 15. A method of providing fail-safe biasing for adifferential communication system, comprising: applying a biasingcurrent and biasing voltage to a first signal line using a first currentsource powered by a first power supply; regulating the biasing currentby driving the first current source with a high-precision voltagereference; coupling a second signal line to a current sink using asecond current source; wherein an electrical path for the biasingcurrent between the first signal line and the second signal line iscompleted by an electrical device coupled to the first signal line andthe second signal line by at least one cable.
 16. The method of claim15, wherein applying the biasing current and biasing voltage furtherincludes providing a first logic level signal, with a controller, toenable the first current source; wherein coupling the second signal lineto the current sink further includes providing a second logic levelsignal, with the controller, to enable the second current source; andwherein the controller is communicatively coupled to the first currentsource and the second current source.
 17. The method of claim 15,wherein applying the biasing current and biasing voltage to a firstsignal line further includes using a third current source powered by thefirst power supply; and wherein coupling the second signal line to thecurrent sink further includes using a fourth current source, wherein thesecond current source and the fourth current source are powered by thefirst power supply.
 18. The method of claim 17, further comprising:coupling the first current source and the third current source inparallel so both the first current source and the third current sourceare configured to provide a portion of the biasing current and biasingvoltage to the first signal line; and coupling the second current sourceand the fourth current source in parallel so both the second currentsource and the fourth current source are configured to receive a portionof a current and a voltage from the second signal line.
 19. The methodof claim 15, wherein the high-precision voltage reference is alsopowered by the first power supply.
 20. The method of claim 15, wherein adifferential signal driver is coupled to the first signal line and thesecond signal line and configured to transmit a differential signalacross the first signal line and the second signal line, wherein thedifferential signal driver is coupled to a second power supply, whereinthe second power supply is de-energized by a controller whenever thefirst power supply is energized, wherein the controller iscommunicatively coupled to the first power supply and the second powersupply.